The present invention relates to a liquid crystal display device comprising thin film transistors, and relates specifically to the structure of the thin film transistor.
In recent years, in the active-matrix liquid crystal display device, studies have been actively carried out on the active-matrix substrates wherein thin film transistors (hereinafter referred to as TFT) are formed in a matrix arrangement on an insulating substrate. Semiconductor materials used for TFT include poly Si (polycrystalline silicon), a-Si (amorphous silicon), Te, and CdSe and others.
An example of the structure of the field effect TFT using a-Si, which is employed in the liquid crystal displaying apparatus, is shown by a planar view in FIG. 4 and a cross-sectional view in FIG. 5. FIG. 5 shows a cross-section taken along line B-B in FIG. 4. A gate electrode 21 of about 1000-about 4000 .ANG. in thickness is formed on a transparent insulating substrate 20 such as a glass substrate, and a gate insulating film 22 of about 100-about 5000 .ANG. in thickness. An a-Si film 23 of about 100-about 2000 .ANG. in thickness, and a protective insulating film 24 of about 1000-about 6000 .ANG. in thickness are laminated thereon consecutively without breaking vacuum by the plasma CVD method. Next, the protective insulating film 24 is patterned, and a P (phosphorus)-doped n.sup.+ -a-Si film 25 of about 100-about 1000 .ANG. in thickness and a source-drain metal film 26 are laminated. Patterning is then performed to form a source electrode 27 and a drain electrode 28 (the protective insulating film 24 is provided to protect the a-Si film 23 from an etchant in performing the n.sup.+ -a-Si film 25 patterning). Furthermore, a pixel electrode 29 is formed in contact with the drain electrode 28. Thus, as is obvious from FIG. 4, a TFT and a pixel are formed at each point of intersection of a gate electrode line 30 and the source electrode 27 in an array shape.
Generally, in the active matrix substrate using TFTs, a TFT at of each point of intersection is driven by a line sequential method. This means that a scanning signal is inputted from one gate electrode line to be scanned, and a data signal is inputted from each source electrode line. The TFT is operated by the respective inputted signals and controls the ON/OFF of a current flowing into the drain electrode. It then selects the state of turn-on or turn-off of the pixel electrode connected to the drain electrode. The total number of TFTs connected t the pixel electrodes is large. For example, there are 87,500 pieces of TFTs in a 250.times.350 matrix. If a defect is produced among a large number of TFTs, a point defect is produced inevitably, and the yield rate of the display device is reduced.
A possible cause of the point defect is that an electrical connection breaks between the drain electrode and the source electrode, or in reverse, an electric short-circuit takes place in the channel part, and thereby the TFT cannot be controlled and normal display cannot be performed.
The TFT, having the above-mentioned structure, has a deficiency in that after patterning to form the source electrode 27 or the drain electrode 28, for example, using Ti, over-etching occurs on the step part (as shown by a mark X in FIG. 4) extending onto the gate electrode line 30. Thus, the resistance of the source electrode 27 or the drain electrode 28 is increased, or a break in the electric connection between the source electrode 27 and the drain electrode 28 is caused, and therefore a point defect is likely to be produced.
The present invention has been achieved in light of the above-mentioned circumstances, and its purpose is to provide a liquid crystal display device capable of improving the yield rate and the reliability of the TFTs.